This invention relates to a device for controlling the charge area of a photoreceptor of an image forming apparatus, such an electrostatic copying machine and the like.
A device for controlling the charge area of a photoreceptor of an electrostatic copying machine or the like has heretofore been disclosed, for example, in an unexamined patent publication of 161342/54. Specifically, the copying machine in which the reflected light from an original is projected to the photoreceptor drum charged by a main charger to form an electrostatic latent image, which is in turn developed by developing means and transferred to a sheet of copying paper by transferring means, is provided with a device for controlling the main charger so that the charge area of the photoreceptor drum corresponds to the size of a sheet of copying paper and waste of toner or pollution of various machine parts is prevented from occurring. In the other words, in such copying machine, toner is attracted to the residual charge on the photoreceptor drum after projecting the image to the drum and the toner image is transferred to the copying paper. However, when the entire surface of the photoreceptor drum is charged by the main charger, the residual charge exists outside an area of the drum corresponding to the copying paper as well after projecting the image to the drum. Then the toner is attracted to an unnecessary area, consequently resulting in the waste of toner and involving dispersion of toner which brings about pollution of parts. The aforementioned device is provided to prevent the undesirable incidents. The device enables proper control of charge area when the image projected area on the photoreceptor drum is in agreement with the size of a sheet of copying paper.
In a copying machine capable of variable magnification and reduction, when copying an original in a reduced size, it frequently occurs that the image projected area on the photoreceptor becomes smaller than the copying paper sheet. However, the conventional device is not able to prevent the incident that residual charge exists in an area which is outside the image projected area on the drum but within the area corresponding to the size of copying paper sheet and the toner is attracted to the unnecessary residual charge.
Consequently, when the toner image is transferred to the copying paper, the problem occurs which the toner is attracted to an unnecessary portion outside the toner image, such as a rear end portion of the copying paper, then resulting in a so-called solid portion. Moreover, charging an unnecessary portion outside the image projected area on the photoreceptor involves deterioration of the photoreceptor.
It is an object of the present invention to provide a device for controlling change area of a photoreceptor which has overcome the above-mentioned drawbacks.
Other objects and advantages of the present invention will become more apparent from the following description.